2 stroke Engine RPM Out of Control...help?

[Wazr Rich]

Hi,

About a year ago I bought a used bike with a kit already installed. Not sure if it's a 49,66, or 80cc because it was painted black and the identiplate on the clutch cover side is pained over and scratched up.

Rode nice for a while but always had an intermittent rpm control problem with it, i.e., at times with the engine being hot (or cold) rpm's would race high and out of control if I didn't stop it by using the kill switch or engaging the clutch to put a load on the motor.

I've got an expansion tube muffler on it and I believe an NT carb (you know the one with the red air cleaner cover). Side note, with the muffler off I looked at the side of the piston and there were visible vertical scratches that I could easily catch my fingernail on. Not good, I know. However, I've seen this kind of scoring before on small 2 stoke pistons but have not experienced high rpms like this thing is doing.

I've read many posts and have found many possible solutions as listed below, some of these I have tried but nothing has worked yet.

Possible Causes
1. low fuel flow. I removed the fuel hose from the carb petcock and checked the fuel flow from tank through the hose and into a bucket. It flowed but I wasn't sure if was sufficient. It seemed like only a trickle. Drained the gas tank and cleaned off the gas tank petcock filter. There was some debris on it. replaced it and the flow check seemed the same.

2. air leak. While engine was running and trying to control the rpms by slightly engaging/disengaging the clutch so it wouldn't run too fast I sprayed carb cleaner around where the carb meets the manifold, manifold meets the intake port, crank drive gear comes through the housing, magneto shaft comes through the case, and all around the case. Never heard the slightest change in rpms while doing this.

3. Main jet clogged. nope, and I lock-tighted it in a few days ago just in case and it's clear with no obstructions.

4. Wrong jet size? It's whatever came in that carb

5. Throttle slide stuck. I was actually looking down the intake side of the carb (with filter off) when the engine was trying to rev itself to death and the slide was as far down as it would go. The throttle opens and closes very freely.

Just scratching my head now so I thought I'd throw this out to anyone who cares to read this kinda stuff. Thanks for any help on this.

Rich

 

[bluegoatwoods]

Hmmm.......weird issue.

But to start with, welcome to the forum. There's a lot of help to be found here.

I'm not really qualified to answer this since I'm drawing a blank on this. As I read your post I was wondering if your throttle cable wasn't damaged or something like that. But you said that the throttle barrel does bottom out in the carb.

From your description, I think that you do not have an NT carb. I'm not sure if that'll matter or not.

Someone will come along with some better guesses. Good luck.

 

[2door]

Welcome to the forum. I hope we can help you with this problem.
It still sounds like an air (vacuum) leak to me. The 'spray test' isn't always conclusive especially if you have a bad crankshaft seal. There are two. One behind the magneto rotor (magnet) and the other behind the primary gear on the right side. Failure of either can give you a fast idle condition as well as other symptoms.

Remove both engine side covers and inspect for oily residue which might indicate a bad seal.

The kit supplied intake manifold gasket is notorious for early failure. It should be replaced with a good automotive quality gasket material and while the manifold is off check the flange for flatness. Every steel manifold I've seen has a warped flange from the welding process. That flange needs to be lapped flush so you have a good gasket sealing surface. The carburetor to intake manifold is another common area for leaks. Any place air can be drawn in downstream of the carb will cause the problem you have.

Are you positive the throttle barrel is returning completely to the idle position? A sticking slide or a binding cable can also be the culprit here. I'd double check that the throttle isn't the problem.

Good luck. Let us know what you find.

Tom

 

[Wazr Rich]

OK, a couple days ago I removed the carb and took a close look at the needle and seat. There may have been debris in there. Anyways I put it back together and although it did a little over reeving it managed to idle for me just fine and I could fine tune it with the idle speed screw to a pretty low rpm. These motors seem finicky at best. I've read posts where a guy said he bought a new carb and it ran like crap for a couple weeks then boom, like magic, something clicked and it ran like a champ. There are significant vertical scores on the piston on the exhaust port side so this engine has seen better days (internally) and I'm thinking that has to have something to do with the way it's running. But it still runs. Got a new kit on the way, should be here today...yippie.

Thanks All for your input

 

[mapbike]

I think everyone else has thoroughly covered all the trouble shooting points so I just want to ring in here and wish you the best with the new engine, I hope its a good one and will run good for you.

Map

 

[Wazr Rich]

OK, I've read so many posts that try to solve a problem but the solution was not really made clear. So, I'll explain what I think happened in this situation that solved the problem.

Problem was over revving, would not idle normal. Finally it started running correctly after rebuilding the carb. However, I did not do anything specific to fix a problem during the carb rebuild. As a matter of fact the carb was super clean inside.

Conclusion:

I think there was some debris that somehow got stuck in the needle and seat thereby restricting fuel flow that caused the rpm over rev condition.

Thanks for all the help and I hope this helps someone else.

 

[mapbike]

OK, I've read so many posts that try to solve a problem but the solution was not really made clear. So, I'll explain what I think happened in this situation that solved the problem.

Problem was over revving, would not idle normal. Finally it started running correctly after rebuilding the carb. However, I did not do anything specific to fix a problem during the carb rebuild. As a matter of fact the carb was super clean inside.

Conclusion:

I think there was some debris that somehow got stuck in the needle and seat thereby restricting fuel flow that caused the rpm over rev condition.

Thanks for all the help and I hope this helps someone else.

Great to hear you got the problem solved.